死锁以及如何解决死锁

心已赠人 2022-03-10 11:04 368阅读 0赞

转自[https://www.cnblogs.com/aflyun/p/9194104.html][https_www.cnblogs.com_aflyun_p_9194104.html]，真的很详细。包括我之前总结的cpu100%如何解决也有。

死锁4大要素：互斥，持有并请求，不可剥夺，持续等待

那么如何避免死锁？破坏其中一个条件

说是这么说，代码撸起来。写一个死锁：

public class TheBackLock {
    
        public static String object1 = "obj1";
        public static String object2 = "obj2";
    
        public static void main(String[] args) {
            Thread thread = new Thread(new MyThread(false));
            Thread thread1 = new Thread(new MyThread(true));
            thread.start();
            thread1.start();
        }
    
    }
    
    class MyThread implements Runnable {
    
        private boolean flag;
    
        public MyThread(boolean flag) {
            this.flag = flag;
        }
    
        @Override
        public void run() {
            if (flag) {
                try {
                    while (true) {
                        synchronized (TheBackLock.object1) {
                            System.out.println(Thread.currentThread().getName() + "lock object1");
                            Thread.sleep(3000);
                            synchronized (TheBackLock.object2) {
                                System.out.println(Thread.currentThread().getName() + "lock object2");
                            }
                        }
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                try {
                    while (true) {
                        synchronized (TheBackLock.object2) {
                            System.out.println(Thread.currentThread().getName() + "lock object2");
                            Thread.sleep(3000);
                            synchronized (TheBackLock.object1) {
                                System.out.println(Thread.currentThread().getName() + "lock object1");
                            }
                        }
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
    
        }
    }

这例子分析下如何死锁了，第一个互斥条件，有两个线程，我们命名为a,b吧。a获取object1锁，synchronized是由jvm进行管理的锁，Thread.sleep(3000)是为了让b有获取object2锁的充足时间。那么3秒过后，a去尝试获取object2锁的时候，这时就互斥了。第二持有并请求，a有了object1锁，再请求object2锁的时候才会死锁。第三不可剥夺 ,synchronized不像Lock类，扩展性强。第四持续等待，看到while(true)了没有

### 如何解决死锁 ###

1.调用jps -l

输出完全的包名，应用主类名，jar的完全路径名

jstack -l pid

查看堆栈信息

jmap也可以dump下来堆栈的信息查看

![20190310213755456.PNG][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70 1][]

deadlock有没有

2.使用jdk->bin下面Visual VM

监控很到位，虽然我不是很会用

3.cpu100%我之前博客说过

[https://blog.csdn.net/weixin\_38336658/article/details/81437022][https_blog.csdn.net_weixin_38336658_article_details_81437022]

总结：不外乎找到占用的pid，转tid，jstack pid，或者jmap  dump出来，查看对应哪里报错

[https_www.cnblogs.com_aflyun_p_9194104.html]: https://www.cnblogs.com/aflyun/p/9194104.html
[20190310213755456.PNG]: /images/20220310/7c92fee224824f04895c3631bf2f7b55.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70]: /images/20220310/256a62f03c414410ac585ff20b5b4fcc.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODMzNjY1OA_size_16_color_FFFFFF_t_70 1]: /images/20220310/7fa9ba6cc6e647edbb56fc341c144b39.png
[https_blog.csdn.net_weixin_38336658_article_details_81437022]: https://blog.csdn.net/weixin_38336658/article/details/81437022